Ion implantation has become a standard, commercially-accepted technique for introducing conductivity-altering impurities into semiconductor wafers. A desired impurity material is ionized in an ion source, the ions are accelerated to form an ion beam of prescribed energy, and the ion beam is directed at the surface of the wafre. The energetic ions in the beam penetrate into the bulk of the semiconductor material and are embedded in the crystalline lattice of the semiconductor material to form a region of desired conductivity. Other processes, including ion beam milling and reactive ion etching, also involve the application of ion beams to semiconductor wafers.
The ion beam utilized in iion implantation and in other ion beam processes is usually a beam of positive ions. The positive ions cause a buildup of positive electrical charges on the workpiece. When the workpiece is conductive, the electrical charges can be removed by electrically connecting the workpiece to a suitable potential, such as ground. However, in the case of nonconducting and low conductivity workpieces such as semiconductor wafers, the positive charges remain on the workpiece and increase in number as the ion beam is applied. Depending on the ion beam current, the duration of the ion implant and the conductivity of the workpiece, a substantial positive potential can build up on the workpiece. Such a potential can damage the highly-sensitive, microminiature devices on semiconductor wafers. Thus, it is important to provide a means for limiting the charge buildup which occurs during ion implantation and other ion beam processes.
Various techniques for neutralization of workpieces during implantation have been disclosed in the prior art. A low energy electron beam directed at a semiconductor device to effect neutralization is disclosed in U.S. Pat. No. 3,507,709, issued Apr. 21, 1970 to Bower. The electron beam carries a negative charge which at least partially offsets or neutralizes the positive charge introduced by the ion beam. This technique is commonly known as electron flooding. The electron beam is produced by a heated filament. One drawback of the disclosed arrangement is contamination of the workpiece by material emitted from the filament. In addition, undesired heating of the workpiece is caused by the heated filament.
Apparatus for neutralizing the potential on a workpiece by directing electrons from a filament into the ion beam is disclosed in U.S. Pat. No. 4,118,630, issued Oct. 3, 1978 to McKenna et al, and U.S. Pat. No. 4,135,097, issued Jan. 16, 1979 to Forneris et al. The disclosed apparatus avoids a direct line of sight between the electron-generating filament and the workpiece. According to these patents, electrons are trapped in the ion beam and are carried with the positive ions to the workpiece. In the disclosed technique, the electrons have relatively high energies and may not be efficiently transported to the workpiece.
A neutralizatoin apparatus wherein a primary electron beam is directed through the ion beam at a secondary electron target positioned adjacent to the ion beam is disclosed in U.S. Pat. No. 4,463,255, issued July 31, 1984 to Robertson et al. Secondary electrons which are stimulated by the primary electron bean have low energies and are efficiently entrapped within the ion beam.
A charge suppressing device which utilizes a magnet for deflecting an electron beam is disclosed in U.S. Pat. No. 4,939,360, issued Jul. 3, 1990 to Sakai. The magnetic field is oriented at an angle to the initial direction of the electron beam and affects the electron beam over a limited portion of the electron beam path.
A flood gun for introducing low energy electrons into an ion beam for neutralizing charge buildup on a semiconductor wafer is disclosed in U.S. Pat. No. 4,825,087, issued Apr. 25, 1989 to Renau et al.
A device for the ionic analysis of an insulating sample wherein the surface of the sample is bombarded by a primary electron beam is disclosed in U.S. Pat. No. 4,564,758, issued Jan. 14, 1986 to Slodzian et al. Negative ions emitted by the sample are used for producing an ion image of the sample. An electron beam whose normal speed component reaches zero at the surface of the sample is directed perpendicularly to the sample through a magnetic prism.
Other techniques for ion beam neutralization are disclosed in U.S. Pat. No. 4,278,890 issued Jul. 14, 1981 to Gruen et al, U.S. Pat. No. 3,622,782 issued Nov. 23, 1971 to Smith et al, U.S. Pat. No. 3,847,115 issued Nov. 12, 1974 to Tashbar and Japanese Patent No. 54-124879 dated Sept. 1979. The neutralization of a target that is irradiated by an electron beam with low energy positive ions is disclosed in U.S. Pat. No. 4,249,077 issued Feb. 3, 1981 to Crawford. A neutralized ion beam propulsion device is disclosed in U.S. Pat. No. 3,363,124 issued Jan. 9, 1968 to Bensussan et al. A scanning electron microscope wherein secondary electrons emitted from a specimen are guided to a detector by a solenoid coil, is disclosed in U.S. Pat. No. 3,474,245, issued Oct. 21, 1969 to Kimura et al.
All of the known prior art techniques for neutralization of workpieces have had one or more disadvantages, including contamination of the workpiece by the electron source, damage to the workpiece by energetic electrons, lack of control over electron energies, insufficient electron beam current to neutralize the workpiece and nonuniform distribution of neutralizing electrons over the surface of the workpiece.
It is a general object of the present invention to provide improved methods and apparatus for ion beam treatment of workpieces.
It is another object of the present invention to provide improved methods and apparatus for neutralization of workpieces in ion beam systems.
It is a further object of the present invention to provide methods and apparatus for directing a high current, low energy electron beam at a semiconductor wafer.
It is yet another object of the present invention to provide charge neutralization methods and apparatus wherein an electron beam is magnetically guided from an electron source to a workpiece.
It is still another object of the present invention to provide methods and apparatus for charge neutralization which minimize contamination of the workpiece.
It is a further object of the invention to provide charge neutralization methods and apparatus wherein electron energies are sufficiently low to avoid damage to the workpiece.
It is another object of the invention to provide charge neutralization methods and apparatus wherein electrons are substantially uniformly distributed over the workpiece surface.
It is a further object of the present invention to provide an ion implantation system including an electron source that is remotely located from the ion beam and means for efficiently transporting a high current, low energy electron beam from the electron source to a target.
It is still another object of the present invention to provide charge neutralization methods and apparatus in ion beam systems wherein an electron source can be repaired or replaced without interrupting the vacuum in the region of the ion beam.